1. Field of the Invention
The present invention is directed to a phase shift mask structure and a method for forming patterns on a semiconductor wafer.
2. Related Art
Conventional processes for the fabrication of semiconductor wafers often incorporate a lithographic process to create a desired pattern on a semiconductor wafer. For example, optical lithography can be used to transfer an integrated circuit pattern through a mask onto a semiconductor wafer, such as a silicon wafer. Conventional masks include chromium binary masks. Openings in the mask permit light from an optical projection unit or light source, such as a laser, to irradiate a photosensitive polymer (such as a conventional photoresist) layer on the surface of the silicon wafer. For example, conventional light sources for optical lithography include ultraviolet wavelength lasers, such as Krypton Fluoride (248 nanometers), Argon Fluoride (193 nanometers), and Fluoride (F2; 157 nanometers) lasers.
One important consideration in the lithographic process is the level of contrast provided on the silicon wafer, where an increased contrast leads to higher resolution. The resolution on the surface of the silicon wafer, in turn, is a function of the wavelength of the imaging light and the numerical aperture of the projection lens utilized to focus the light onto the silicon wafer. Another factor in increased resolution is the depth-of-focus, which corresponds to the lithographic system""s tolerance towards imperfection in wafer flatness. As a result of these relationships, current trends are moving towards increased numerical aperture lenses and imaging light having shorter wavelengths. Another approach is utilizing a phase-shift mask (xe2x80x9cPSMxe2x80x9d) or xe2x80x9cphase-shifting photomaskxe2x80x9d which results in higher contrast on the surface of the silicon wafer.
PSMs are becoming increasingly prevalent in the effort to extend the useful life of optical lithography. The PSM allows for increased pattern resolution in comparison to chromium binary masks. The increased resolution is achieved by shifting the phase of the light at feature boundaries 180 degrees relative to one another. This phase shift causes destructive interference at the feature boundary sharpening the feature image. See e.g., Garcia, et. al, xe2x80x9cThin Films for Phase-shift Masksxe2x80x9d, Vacuum and Thinfilm Magazine, September 1999, pg 14-21.
Currently there are two major approaches to constructing a phase shift mask: (1) alternating-aperture type (or xe2x80x9cLevensonxe2x80x9d) masks and (2) Embedded Attenuated Phase-shift masks (xe2x80x9cEAPSMsxe2x80x9d).
A conventional alternating-aperture type PSM consists of a quartz substrate that has been etched in various portions that correspond to the pattern to be imaged on the wafer.
Alternatively, EAPSMs can also be utilized. The EAPSM allows exposing light or ultra violet (UV) radiation to pass through all areas that are to be exposed on the wafer while simultaneously allowing a small fraction of light or UV radiation through in the nonexposed areas. In the areas that are to remain unexposed on the wafer, the light is shifted 180 degrees out of phase, and attenuated in intensity (to approximately 5% to 15% of the incoming intensity). This approach allows all features to be exposed or printed (by the unshifted light) with sharper edges due to destructive interference with the attenuated phase shifted light.
A common strategy for fabricating EAPSMs is to use either a single layer or multi-layer stack of phase shifting materials to shift and attenuate the light. For example, a conventional EAPSM has a structure similar to that shown in FIG. 1. EAPSM 10 has a transparent quartz substrate 12 and a shifter material layer 14.
Conventional phase shifter materials utilized for phase shifter layer 14 include MoSixOyNz, SixNy or CrOxFy. These materials are often utilized because the main chemical constituent, either silica, silicon nitride or chrome fluoride, is sufficiently transmissive (5%-20%) at the operating lithographic wavelength. Recent design trends have also opted toward multi-layer films where an optically transparent film is layered with an optically absorbing film. Some of the published multi-layer phase shifting films include SiN/TiN, MoSiON, CROF composites and, more recently, ZrSiO composites. See e.g., Onodera, et. al, xe2x80x9cZrSiO: a new and robust material for attenuated phase-shift mask in Arxe2x80x94F lithographyxe2x80x9d, 19th Annual Symposium of Photomask Technology, September 1999, pg. 337-340.
Conventional processing of an EAPSM, such as EAPSM 10, which occurs as follows. A mask/resist is formed on phase shifter layer 14. A masking layer is patterned by lithography. Then, portions of phase shifter layer 14 are removed by a separate etching process using the masking layer as a mask. One conventional technique is to etch the shifter layer utilizing a Halogen-based, fluorine-based radio frequency (xe2x80x9cRFxe2x80x9d) plasma etching.
The inventors have realized that the Halogen-based, fluorine-based RF plasma etching can also damage and/or remove a portion of the underlying quartz substrate, because such etching has low selectivity between the phase shifting layer and the substrate. This can adversely affect the phase-shift properties of the EAPSM.
In view of the foregoing, it would be desirable to provide an improved EAPSM structure that provides near optimal 180 degree phase shifting and a process for fabricating the EAPSM structure that does not damage the substrate layer of the EAPSM. According to one embodiment of the present invention a phase shift mask, capable of shifting a phase of incident light and/or ultra violet radiation 180 degrees, comprises a transparent substrate, an etch stop layer disposed on the substrate, and a layer comprising a phase shifting material disposed on the etch stop layer. The etch stop layer can comprise a material having a different etch chemistry from the substrate and from the phase shifting material.
According to another embodiment of the present invention, a process for fabricating a phase shift mask structure comprises providing a transparent substrate containing an etch stop layer over the substrate, a phase shifting layer comprising a phase shifting material over the etch stop layer, and a masking layer over the phase shifting layer. The process also comprises etching the phase shifting layer using the masking layer as a mask such that the etching stops on the etch stop layer. Further, in this embodiment, the etch stop layer can be engineered so that it can be etched with a plasma etch that includes plasma chemistries that are highly selective to the substrate.
According to another embodiment of the present invention, an in-process phase shift mask structure, capable of shifting the phase of incident light and/or ultraviolet radiation 180 degrees, comprises a transparent substrate, an etch stop layer disposed on the substrate, a layer comprising a phase shifting material disposed on the etch stop layer, a hard mask layer disposed on the phase shift material layer, and a resist layer disposed on the hard mask layer.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.